Conventional cotton harvesters include two or more harvesting units commonly supported for vertical movement on a fore-and-aft wheeled frame of the harvester. Each harvesting unit includes a harvesting assembly defining a plant passage and a harvesting mechanism arranged within the housing. As the cotton harvester is driven across the cotton field, a row of cotton plants passes through the passage, and the harvesting mechanism removes the cotton therefrom. Cotton grows along the entire height of the cotton plant. At the lower end, the cotton grows barely off the ground and sometimes on the ground. The harvesting units, therefore, follow as close to the ground as possible so as to pick all of the cotton from the plants.
The ground over which the harvester is driven is usually uneven. Accordingly, if the harvesting unit is set for a lowermost point of depression on the ground, stalk lifters extending from a forward end of the harvesting unit will tend to "dig into" high points of ground contour. As the harvester is driven across the field, the wheels on the harvester frame ride between adjacent rows of cotton plants. In softer muddy conditions, the wheels of the harvester furthermore deform the field into slight recesses and valleys or raised ridges. As will be appreciated, proper positioning of the harvesting unit relative to the ground contour is further complicated in such undulating field conditions.
To optimize efficiency during the harvesting operation, cotton harvesters are known to include a harvesting unit height control system for automatically controlling the height of the harvesting unit relative to the ground contour. The elevation of the harvesting unit is primarily controlled by a lift mechanism actuated in accordance with ground contours. Variations of the ground contour are sensed by a ground engaging element, such as a shoe, mounted on the harvesting unit in a manner to press on the ground and be positionally displaced in response to variations of the ground contour profile.
The sensing shoe of the height control system is connected to a control valve of a hydraulic system connected to the lift mechanism. The control valve controls operation of the lift mechanism and, thereby, elevation of the harvesting units. As is conventional, the control valve moves from a "Neutral" position to either "Lower" or "Lift" positions in response to vertical displacement of the ground engaging element or shoe. A conventional control valve includes a reciprocal spool valve movable through a predetermined range of travel.
A vertically adjustable linkage assembly typically interconnects the ground engaging element and the control valve. The vertical adjustment of the linkage assembly allows the "Neutral" position of the ground engaging element or shoe to be vertically adjusted relative to the harvesting unit and within a limited range of movement.
To avoid maintaining the hydraulic system of the harvester "active", the harvesting units are often set upon the ground when there is a "break" or upon completion of a harvesting operation. A major problem encountered with known height control systems is that the "Neutral" position of the sensing element or shoe may be vertically positioned relative to the harvesting unit such that vertical displacement of the shoe after the harvesting unit is set upon the ground is greater than the displacement allowed for the valve spool of the control valve. As will be appreciated, overtravel of the shoe beyond the operating range of the spool valve can result in damage to the components of the height control system. Accordingly, the "Neutral" position adjustment of known height control systems is severely limited.
To promote proper operation of the lift mechanism, the height control system must be responsive to changes in ground contours. In this regard, another problem with known height control systems involves the weight of the linkage assembly used to interconnect the ground engaging shoe with the control valve. Although the shoe of the height adjusting system can properly operate on a firm portion of the field, the bulkiness or weight of the linkage assembly detracts from the responsiveness of the sensing shoe and tends to cause the shoe to "dig in" at soft portions of the field with attendant undesirable consequences.